1. Field of the Invention
The present invention relates to drill bits and more particularly to improved cutter geometries for fixed cutter drill bits and cutters and drill bits incorporating the same.
2. Background Art
Fixed cutter drill bits are widely used in the petroleum and mining industry for drilling wellbores through earth formations. The bits typically include a bit body with a threaded connection at a first end for attaching to a drill string and cutting structure formed at an opposite end for drilling through earth formation. The cutting structure typically includes a plurality of blades that extend radially outwardly from a longitudinal axis of the bit body. Ultrahard compact cutters are typically mounted in sockets formed in the blades and affixed thereto by press fitting or brazing. Fluid ports also may be positioned in the bit body to distribute fluid around the cutting structure of the bit and flush formation cuttings away from the cutters and borehole bottom during drilling.
Cutters used for fixed cutter drill bits typically comprise ultrahard compacts which include a layer of ultrahard material bonded to a substrate of less hard material through a high pressure/high temperature (HP/HT) sintering process, a brazing process, mechanical locking, or other means known in the art. Cutters are conventionally cylindrical in form with circular cross sections.
In mounting cutters on a bit a trade off exists between the depth of cutter setting into the bit body and the remaining cutter exposure available for drilling. Cutters are typically mounted with only about one-half of the cutter body exposed for drilling, with the other half being brazed into a socket formed in the bit body. For drilling applications where cutters may become exposed to high impact loads, such as in drilling rock formations tough in shear or in high speed drilling applications, more than half of the cutter body surface may be brazed into the cutter socket to provide sufficient braze strength for retaining the cutters in place during drilling. However, this deeper setting reduces the amount of cutter exposure remaining for drilling.
As cutters wear during drilling, an ever increasing wear flat forms at the cutting edges which increasingly slows down the rate of penetration (ROP) of the bit and increases the weight on bit required to maintain drilling. As the size of the wear flat increases, the heat generated at the cutting edge also increases and the ability of the drilling fluid to cool and clean the cutter decreases. The drilling life of a bit (bit life) is frequently limited by the amount of wear the cutters can experience before the displaced formation continuously interferes with the outer surface of the bit body and greatly retards the drilling rate. For conventional cutters, this wear amount is normally less than one-half of the cutter's diameter.
In many applications, conventional cutters do not provide the desired clearance between the cutting edge and the supporting bit body surface to prolong bit life. Also, because of the limited stand-off provided by conventional cutters, sufficient cooling and cleaning of the cutters may not be accomplished, especially when the entire exposed portion of a cutter becomes embedded in the earth formation leaving no room for drilling fluid to flush across the cutting face.
To overcome deficiencies noted for conventional cutters, elliptical cutters have been proposed as disclosed in PCT Publication No. WO 9214906 (Simpson et al). Elliptical cutters can be mounted on a bit with their major axes projecting outwardly from the bit body to provide increased cutting edge extension from the bit body surface. One problem associated with elliptical cutters is that their narrow cutting tips make them more susceptible to impact fracture during drilling, especially when exposed to higher impact loads, such as those associated with harder formation and higher speed drilling. Elliptical cutters are also significantly more difficult and expensive to manufacture than conventional circular cutters. Additionally, in many applications, the drilling life of the bit is still limited by the amount of wear the cutters can experience before formation continuously interferes with the bit body and greatly retards the drilling rate.
Asymmetric cutters have also been proposed as disclosed in U.S. Pat. No. 5,383,527 (Azar). These asymmetric cutters include a cylindrical base portion at one end and an asymmetrical cutting face at the other end which projects beyond the wall of the base portion towards a surface to be drilled. Asymmetric cutters advantageously provide broader cutting tips and a larger diamond volumes at the cutting face exposed for drilling than an elliptical cutter of equivalent extension. However, the geometry of the proposed asymmetric cutters also makes them more difficult and expensive to manufacture than conventional circular cutters.
Accordingly, a cutter geometry providing increased bit life, especially for use in harder formation and/or high speed drilling applications, along with reduced difficulty and/or expense in manufacture is desired.